1. Field of the Invention
This disclosure relates to a sheet stacking apparatus, a sheet feeding apparatus, and an image forming apparatus.
2. Description of the Related Art
Nowadays, in image forming apparatus such as copying machines, printers, and facsimiles, those configured to form images on a sheet fed from a sheet feeding apparatus by an image forming portion is widely used. The sheet feeding apparatus is generally configured to demountably mount a sheet feeding cassette as a sheet storage portion configured to store sheets in an apparatus main body, and feed the sheets stored in the sheet feeding cassette by a pickup roller provided on the apparatus main body automatically.
Examples of the sheet feeding apparatus of this type include those including a tray configured to be moved upward and downward by an elevating unit so as to feed the sheets stacked on the tray of the sheet feeding cassette by pressing with a pickup roller. In addition, the sheet feeding cassette is slidably provided with a trailing end regulating unit configured to regulate positions of upstream ends of the sheets stacked on the tray in the sheet feeding direction (hereinafter, referred to as a trailing end) so as to allow sheets of different sizes to be stored. In addition, the sheet feeding cassette is provided with a pair of side end regulating units configured to regulate side end positions of the sheets stacked on the tray in a direction orthogonal to a sheet feeding direction (hereinafter, referred to as a width direction).
When feeding the sheet, side ends of the sheets on the tray are regulated by the pair of side end regulating units, and trailing ends of the sheets on the tray are regulated by the trailing end regulating unit, so that the positions of the sheets are regulated at predetermined positions. Thereafter, the tray is moved upward by the elevating unit, and the pickup roller is pressed against the sheets stacked thereon, and rotates to feed the sheets.
Examples of the sheet feeding apparatus of the related art include a type configured to feed sheets having an uneven thickness such as envelopes. Examples of the sheet feeding apparatus of this type include those configured to be provided with a specific middle plate and press the sheets stacked on the middle plate from above by a press roller in order to stack a larger amount of the sheets having an uneven thickness as disclosed in Japanese Patent Laid-Open No. H11-35175. When feeding the sheets, the middle plate is pressed by the pickup roller provided above with a spring, and the pickup roller is rotated while pressing the sheets by the press roller to feed the sheet.
In the sheet feeding apparatus of the related art having the configuration as described above, sheet conveying properties are improved to some extent by pressing a bundle of sheets having an uneven thickness with the spring or the roller to bring postures of the sheets horizontal. However, in the case where the sheets are envelopes, each envelope is provided with a flap 23 for closing an opening as illustrated in FIG. 9. Since the shape of this flap 23 does not have a shape covering an entire surface of an envelope P but has a shape covering part of the surface of the envelope P, the thickness of a portion of the envelope P covered by the flap 23 becomes thicker than other portions. In other words, the envelope P is formed by folding a sheet into a bag shape and, in addition, the flap 23 is further folded at an end, so that the envelope P includes a portion having a double thickness of the sheet and a portion having a triple thickness of the sheet.
Therefore, when stacking the envelopes P in the same orientation, the height H1 on a side where the flaps 23 are located is higher than a height H2 on the side where the flaps 23 are not located. The larger the number of the envelopes P to be stacked, the larger the difference between the heights H1 and H2 of the bundle of the envelopes P becomes, so that a topmost envelope P1 is inclined significantly.
FIG. 10 illustrates a state in which the number of the envelopes P to be stacked is large, and the topmost envelope P1 is significantly inclined in accordance with an increase of the number of the stacked envelopes P. In FIG. 10, reference numeral 100 denotes a pickup roller configured to feed the envelope, and reference numeral 108 denotes a middle plate capable of moving upward and downward. The middle plate 108 is controlled to move upward and downward in accordance with the height of the pickup roller 100. Reference numerals 110 and 111 denote side regulating plates configured to regulate the side end positions of the envelopes P. Reference numeral 114 denotes a side wall on the downstream side of the storage, not illustrated, configured to store the envelopes P in the feeding direction.
When the topmost envelope P1 is inclined, the pickup roller 100 comes into abutment with an upper surface of the topmost envelope P1 at only one end of the pickup roller 100 in the width direction, that is, a state of so-called one-side abutment, and hence cannot come into abutment with the upper surface of the topmost envelope P1 uniformly. In this case, a feeding force of the pickup roller 100 is not transmitted to the envelope P1 uniformly, so that a feed error due to slippage or skew caused by the one-side abutment may occur. Since the topmost envelope P1 is inclined, a portion of the topmost envelope P1 on the lower side is located on the lower side of the side wall 114, and if the envelope P1 is fed in this state, the envelop P1 abuts against the side wall 114 and hence cannot be fed. In this manner, when a number of the envelopes are stacked, the difference in height in the stacking direction of the envelopes is increased. Therefore, there is a problem that the envelope cannot be fed reliably.